Field of the Invention
The present invention relates to an AF controller used in an optical device, such as a television lens and a video lens, and more particularly, to an AF controller used in an optical device having an auto focus function including a focus state detector of a phase difference detection system, a lens apparatus including the AF controller and an image pickup apparatus.
Description of the Related Art
Conventionally, there is an AF adjustment method of controlling the position of a focus lens to continue automatically focusing an object all the time in video imaging (hereinafter, called “full-time AF”). However, an object not intended by the photographer may be focused in the full-time AF, depending on the imaging conditions. For example, the focal position may be moved to the background during imaging of a main object. To solve the problem, Japanese Patent Application Laid-open Mo. 2008-164832 discloses a technique of setting a limit to the driving range of the focus lens according to the movement of the object and driving the focus lens only in the limited range.
In an imaging node, the focus needs to be switched from a focused object to an object at a position with a different object distance due to a change in the composition. However, the driving lens of the focus lens is limited in the technique disclosed in Japanese Patent Application Laid-open No. 2008-164832, and focusing the object at a different object distance is difficult. More specifically, although the same object can be tracked, handling the change in the object due to the change in the composition is difficult. Therefore, there can be a method of not setting a limit to the driving range in order to focus the object when the object is changed due to the change in the composition. However, although the change in the composition can be handled in the method of not setting a limit to the driving range, an object not intended by the photographer may be focused. Therefore, there is a method of using a plurality of focus detection sensors to select a main object from detection results after waiting for completion of charge accumulation by all of the focus detection sensors. The charge accumulation time of the focus detection sensors varies depending on the luminance of the target object, and it takes more time for a focus detection sensor targeting an object with a low luminance to accumulate the charge compared to other focus detection sensors. More specifically, in the method of waiting for the completion of the charge accumulation by all of the focus detection sensors, the possibility of focusing an object other than the main object can be reduced, and the stability improves. However, much time is required, and the tracking performance is reduced. Therefore, to improve the tracking performance, there can be a method of adopting the detection result of the focus detection sensor that has first completed the charge accumulation, and the charge accumulation of the other focus detection sensors is forcibly terminated. In the method of adopting the focus detection sensor that has first completed the charge accumulation, the change in the composition can be handled, and the tracking performance improves. However, when there is an object with a high luminance in the background, the focal position may be moved to the object with a high luminance in the background.
FIG. 15A (composition 1) illustrates a main object and sensor areas formed by a plurality of focus detection sensors. An inner frame of FIG. 15A (composition 1) indicates an AF frame for setting sensor areas used to detect the focus, and three areas divided by dotted lines in the AF frame indicate sensor areas A, B and C. The main object is focused in FIG. 15A (composition 1). FIG. 15A (composition 2) illustrates a case in which the background includes an object with a higher luminance than the main object after the composition is changed from FIG. 15A (composition 1). FIG. 15B illustrates a timing chart of AF processing in the composition of FIG. 15A (composition 2). In the process of FIG. 15B, a sensor area in which the charge accumulation is first completed is adopted, and the charge accumulation of the other sensor areas is forcibly terminated to improve the tracking performance. At time t0, the charge accumulation of the sensor areas A, B and C is started. At time t1, the charge accumulation of the sensor area A is completed, and the charge accumulation of the sensor areas B and C is forcibly terminated. A defocus amount is derived based on the detection result of the sensor area A in which the charge accumulation is first completed, and a focus lens target position is derived based on the defocus amount. The focus lens is then driven based on the focus lens target position. As a result, an object in the background that is not the main object is focused. In this control method, the processing time is short, and the tracking performance is high. However, an object not intended by the photographer may be focused, and the stability is reduced. If all sensor areas in the AF frame are detected to improve the stability, the tracking performance is reduced.
An object of the present invention is to provide an AF controller that can attain both of AF tracking performance and stability, a lens apparatus including the AF controller and an image pickup apparatus.